Apparatus for producing electron microscope and diffraction images separately and simultaneously on the image plane



Dec. 21, 1965 SHINJIRO KATAGlRl ETAL 3,225,192

APPARATUS FOR PRODUCING ELECTRON MICROSCOPE AND DIFFRACTION IMAGES SEPARATELY AND SIMULTANEOUSLY ON THE IMAGE PLANE Filed Dec. 5, 1963 RECTANGULAR WAVE OSCILLATOR CURRENT STABILIZER FIG. 3

INVENTOREE Skiujro au-a ar. sqsumu OZQSQ- United States Patent Office 3,225,192 Patented Dec. 21, 1965 3,225,192 APPARATUS FOR PRODUCING ELECTRON MICROSCOPE AND DIFFRACTION IMAGES SEPARATELY AND SIMULTANEOUSLY ON THE IMAGE PLANE Shinjiro Katagiri, Kodaira-shi, and Susumn Ozasa, Hachioji-shi, Japan, assignors to Kabushiki Kaisha Hitachi Seisakusho, Tokyo-to, Japan, a joint-stock company of Japan Filed Dec. 5, 1963, Ser. No. 328,247 Claims priority, application Japan, Dec. 28, 1962,

37/ 59,176 i 4 Claims. (Cl. 250-495) This invention relates to electron microscopes, and more particularly it relates to a new and original method and means in an electron microscope by which it is possible to observe simultaneously on one fluorescent screen a selected area image and the electron diffraction image corresponding thereto.

It is a principal object of the present invention, in its broad aspect, to provide a technique and means relating to electron microscopes by which it is possible to observe rapidly the electron diffraction image of a desired viewing field or the electron microscope image corresponding to a desired electron diffraction image, thereby to facilitate greatly various work such as study of crystalline specimens.

It is another object of the invention to overcome certain difficulties encountered in the prior art as will be described hereinafter.

The foregoing object and other objects and advantages as will presently become apparent have been achieved by the present invention, the nature, principle, and details of which will be best understood by reference to the following description, taken in conjunction with the accompanying drawings in which like parts are designated by like reference characters, and in which:

FIGURE 1 is a schematic line drawing indicating the system of electron optics of an electron microscope of known type;

FIGURE 2 is a schematic line drawing indicating the arrangement of a preferred embodiment of the electron microscope according to the invention;

FIGURE 3 is a graphical representation indicating the operation of the electron microscope shown in FIGURE 2;

FIGURE 4 is a schematic line drawing indicating the compositonal arrangement of another preferred embodiment of the invention; and FIGURE 5 is a graphical representation indicating the operation of the electron microscope shown in FIGURE 4.

In order to indicate fully the nature and utility of the present invention, the following brief consideration of prior electron microscopes is believed to be necessary.

Heretofore, in the case when a selected area diffraction image was to be obtained in an electron microscope, the common practice has comprised first moving the desired field into a field limiting aperture and observing the said field, then adjusting the focal length of the lens for electron diffraction, and forming an electron diffraction image. For example, as shown in FIGURE 1, in one example of a conventional electron microscope comprising an electron source 1, and an anode 2, a condenser lens 3, an objective lens 5, a field limiting aperture means 6, and a diffraction lens 7 (generally called an intermediate lens since, in general, it is disposed in a position intermediate between the projection lens and the objective lens) and prepared for examination of a specimen 4, the image of points A and B in the specimen plane is formed at points 'A' and B as shown in FIGURE 1 in the plane of the field limitingaperture 6. Of the image so formed by the objective lens, only that portion within the field limiting aperture appears in the field of vision in the object plane of the intermediate lens 7. Therefore, if the object plane of the intermediate lens is made to coincide with the field limiting aperture 6, an image due to the intermediate lens 7 will be formed as A" and B" in an image plane 8.

Next, if, with the focal length of the objective lens 5 maintained constant, the focal length of the intermediate lens 7 is increased, and the image plane 8 and the conjugate object plane are coincident with the back focal plane of the objective lens 5, the electron diffraction image a, b of the specimen 4 formed in the back focal plane can be projected on the image plane 8, as is well known. The electron diffraction image a, b so projected corresponds to the image A", B". These images a, b and A", B are further enlarged, if necessary, by means of a projection lens.

The above description relates to the principle of the selected area diffraction technique as known heretofore. However, as described above, this technique requires adjustment of focusing of the intermediate lens 7 for each field. Therefore, in a known instrument of this type, when an electron diffraction image is being observed, the corresponding electron microscope image cannot be ob served simultaneously. Conversely, the reverse and simultaneons accomplishment of these observations is impossible. Accordingly, in the case when, in an electron microscope, it is desired to observe crystals or precipitations within metals, and it is desired to analyze the said crystals or precipitations, since the specimens to be observed are generally numerous, the aforesaid focus adjustment of the diffraction lens for each field is an extremely tedious procedure. Moreover, the corresponding image during the photographing of the electron diffraction image cannot be varied.

In view of the above inconveniences, the present invention contemplates overcoming the difficulties encountered heretofore by providing a method and means whereby, by causing the focal point of a lens capable of projecting an electron diffraction image in an equivalent manner to vary periodically, an electron diffraction image and an electron microscope image are caused to appear simultaneously on one fluorescent screen, and observation of both images can be accomplished rapidly. The means thus provided by the present invention is highly effective for use in research on crystalline materials, particularly research in which electron microscopes for thin foils of metal are utilized.

The specific nature and details of the invention will be more fully understood from the following description thereof with respect to preferred embodiments of the in vention as applied to an electron microscope of magnetic field type.

Referring to FIGURE 2, the essential feature of the electron microscope is the creation of an electron diffraction image and the electron microscope image corre sponding thereto on one and the same image plane by causing, by means of a current stabilizer 1t) and a rectangular wave oscillator 12, the focal length of a lens 7 (intermediate lens) for electron diffraction to vary periodically.

More specifically, first, the exciting current for the intermediate lens 7 supplied from the current stabilizer is regulated to cause an electron diffraction image a, b, to be projected at a, b on an image plane 8. Next, by means of the rectangular wave oscillator 12, the current stabilizer is controlled to cause the exciting current for the intermediate lens 7 to assume the form of a rectangular wave, and adjustment is made so that, at the maximum value of this current, the image A, B is projected on the A", B" plane. Thus, the objectof simultaneously projecting the diffraction image and the electron microscope image on the image plane 8 is achieved.

Since the two images so produced are overlapping, distinguishing one from the other may be diflicult. In such a case, this difficulty is overcome by providing an electromagnet 9 for electron beam deflection below the intermediate lens 7 and supplying to this electromagnet 9 an exciting current of rectangular wave form from a current stabilizer 11 controlled by the aforementioned rectangular wave oscillator 12, the said exciting current being supplied simultaneously with the supplying of exciting current to the intermediate lens 7. The electron microscope image in the image plane 8 can then be formed at a position such as A", B" which is not superimposed on the electron diffraction image a, b.

FIGURES 3(1') and 3(1'1') respectively indicate the exciting currents supplied to the intermediate lens 7 and the electromagnet 9. The rectangular wave current (i) for the intermediate lens 7 has a value C for electron diffraction and a value D for the electron microscope. By selecting a rectangular wave frequency of 20 cycles/ sec. or higher, steady images can be viewed in both cases. The rectangular wave current (ii) for the electromagnet 9 need not flow during the observation of the electron diffraction image. That is, shutting off the electromagnet current during this image observation will not, of course, obstruct the attainment of the original objects of the present invention.

For the generation of rectangular wave signals of the instant apparatus, it is also possible to obtain control by means of mechanical means, instead of electrical means which are ordinarily widely used. However, transient phenomena occurring during increasing and decreasing of currents and their on and off switching cause irregularities in their rectangular waveform, thereby causing deteri oration of the quality of the images. For this reason, it is necessary to use current stabilizers having excellent transient characteristics.

On one hand, in the case when the diameter of the field limiting aperture is made large for the purpose of projecting the diflraction image, the spherical aberration of the intermediate lens causes the diffracted spot or the width of the diffracted ring to increase. Therefore, it is necessary to avoid the use of a lens having a large spherical aberration. For the same focal length, the coeflicient of spherical aberration decreases in approximately inverse proportion to the aperture diameter of the lens pole pieces. On the other hand, the exciting current of the lens increases in approximately direct proportion to the said aperture diameter. Accordingly, in attempting to decrease the spherical aberration, it is difficult to avoid an increase in the exciting current, that is, an increase in the required ampere-turns. For this reason, even if current stabilizers having excellent transient characteristics as mentioned above are used, there is a great possibility, because of the foregoing reason, of an increase in the transient phenomena. In actual practice, therefore, it is believed to be advantageous and desirable to provide counter-measure means.

The present invention, in another aspect thereof, provides such means as illustrated by the example shown in FIGURE 4. In the electron microscope shown, an auxiliary lens 13 is provided below the intermediate lens 7, and, according to necessity, an electron beam deflecting magnet 9 is provided below the auxiliary lens 13 as in the aforedescribed examples. The exciting current for the intermediate lens 7 is selected to be suitable for forming a diffraction image on the image plane 8, and for the exciting current for the auxiliary lens 13, a rectangular wave current is applied. Adjustments are so made beforehand that, when the currents are applied, the microscope image due to the compound lens consisting of the intermediate lens 7 and the auxiliary lens 13 will be formed on the image plane 8. In this case, by causing the exciting current (iv) for the electromagnet 9 and the exciting current (iii) for the auxiliary lens 13 to have the same period as indicated in FIGURE 5, the two images can be prevented from overlapping on the image plane 8.

Thus, although in the case of the diffraction image, the effect of the spherical aberration is great, at the time of observation of the microscope image, the quality of the image under the influence of spherical aberration is not highly problematical. Accordingly, by the above described arrangement, the auxiliary lens 13 functions pri marily as a lens for enlarging the microscope image. Therefore, an auxiliary lens 13 of small aperture diameter can be used. As a result, there is no necessity of varying a large exciting current as in the case of the intermediate lens, and, the aforementioned problem of transient phenomena being thus solved, it becomes possible to observe both diffraction and microscope images of excellent quality.

Although in the above-described example, the rectangular wave current for the auxiliary lens 13 has been described as being so adjusted that the compound lens consisting of the intermediate lens 7 and the auxiliary lens 13 operates only at the time of current application, such an operation is not always necessary. That is, the required condition is only that the magnitude of the said current be periodically varied, and that at its maximum and minimum value the microscope image and the diffraction image be respectively projected on the image plane 8.

Furthermore, the maximum value D of the current (iv) indicated in FIGURE 5, in the case wherein the auxiliary lens is added, need not always be such as to cause the image A, B to be precisely projected in the plane A, B". The reason for this is that in an electron microscope, since the aperture angle of the image forming electron beam is extremely small, the depth of focus is great, and a somewhat out-of-focus condition does not have a critically adverse effect for recognizing the configuration of the image. That is, even if the compound focal length of the auxiliary lens 13 and the intermediate lens 7 is somewhat less than the focal length at the time of projection of the electron diffraction image, there will be no adverse effect in actual practice if a magnification of a degree such as to afford recognition of the configuration of the image is applied.

While the disposition of the auxiliary lens 13 on the specimen side of the intermediate lens '7 will have no adverse effect for reasons of principle, its disposition at a position as remote as possible from the field limiting aperture 6 is more advantageous on the point of minimizing the required ampere-turns.

Although the foregoing description has been limited to electron microscopes of magnetic types, the present invention may be applied to cases wherein static type lenses are used in place of magnetic type lenses, and static type deflecting plates are used in place of electron beam deflecting electromagnets, to produce equivalent results. In such a case, however, while there is no possibility of occurrence of transient phenomena due to causes such as exciting coil inductance in a microscope of magnetic type, it is necessary to provide means making possible the change of applied high voltage into a rec tangular wave form, which necessity presents a technical difficulty. Of course, in the case of a magnetic type microscope, the use of a static type deflecting plate in place of the aforementioned electron beam deflecting electromagnet is especially advantageous on the point of overcoming difficulty due to transient phenomena.

From the foregoing disclosure it will be apparent that, in the electron microscope according to the present in vention, by periodically changing into rectangular wave form the exciting current for a lens (intermediate lens or auxiliary lens) capable of projecting an electron diifraction image formed on the back focal plane of an objective lens, simultaneous observation on a fluorescent screen of the electron diffraction image and the corresponding electron microscope image is made possible. It is to be observed, therefore, that the present invention aflords highly convenient and efficient features in an electron microscope.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In an electron microscope of the type provided With at least a source of electrons, an anode, a condenser lens, means for supporting a sample, an objective lens, a field limiting aperture, an intermediate lens, and a fluorescent screen disposed in the order named, the improvements which comprise, in combination, first means to form al ternately a microscope image and a diffraction image by varying periodically the focal length of said intermediate lens, and second means to cause only the deflection of an image forming electron beam of said microscope image at the image side of said intermediate lens in synchronism with the periodic variation of said focal length, thereby producing separately and simultaneously said microscope image and diffraction image on said fiuorescent screen within the same visual field.

2. The electron microscope as defined in claim 1, wherein said first means consist of a first current stabilizer connected to said intermediate lens and a rectangular wave oscillator connected to said first current stabilizer; and said second means consist of an electr-omagnet disposed between said intermediate lens and said flu0rescent screen, a second current stabilizer connected to said electromagnet, and said rectangular wave oscillator connected to said second current stabilizer.

3. In an electron microscope of the type provided with at least a source of electrons, an anode, a condenser lens, means for supporting a sample, an objective lens, a field limiting aperture, an intermediate lens, and a fluorescent screen disposed in the order named, and an auxiliary lens provided in the vicinity of said intermediate lens, the improvements which comprise in combination, first means to form alternately a microscope image and a diffraction image by varying periodically the focal length of said auxiliary lens, and second means to cause only the defiection of an image forming electron beam of said microscope image at the image side in synchronism with the periodic variation of said focal length, thereby producing separately and simultaneously said microscope image and diifration image of said fluorescent screen within the same visual field.

4. The electron microscope as defined in claim 3, wherein said first means consist of a first current stabilizer connected to said auxiliary lens and a rectangular wave oscillator connected to said first current stabilizer; and said second means consist of an electromagnet disposed between said auxiliary lens and said fluorescent screen, a second current stabilizer connected to said electromagnet, and said rectangular wave oscillator connected to said second current stabilizer.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner. 

1. IN AN ELECTRON MICROSCOPE OF THE TYPE PROVIDED WITH AT LEAST A SOURCE OF ELECTRONS, AN ANODE, A CONDENSER LENS, MEANS FOR SUPPORTING A SAMPLE, AN OBJECTIVE LENS, A FIELD LIMITING APERTURE, AN INTERMEDIATE LENS, AND A FLUORESCENT SCREEN DISPOSED IN THE ORDER NAMED, AND A FLUORESCENT WHICH COMPRISE, IN COMBINATION, FIRST MEANS TO FORM ALTERNATELY A MICROSCOPE IMAGE AND A DIFRACTION TO FORM ALVARYING PERIODICALLY THE FOCAL LENGTH OF SAID INTERMEDIATE LENS, AND SECOND MEANS TO CAUSE ONLY THE DEFLECTION OF AN IMAGE FORMING ELECTRON BEAM OF SAID MICROSCOPE IMAGE AT THE IMAGE SIDE OF SAID INTERMEDIATE LENS IN SYNCHRONISM WITH THE PERIODIC VARIATION OF SAID FOCAL LENGTH, THEREBY PRODUCING SEPARATELY AND SIMULTANEOUSLY SAID MICROSCOPE IMAGE AND DIFFRACTION IMAGE ON SAID FLUORESCENT SCREEN WITHIN THE SAME VISUAL FIELD. 